Rickettsia tsutsugamushi, the causative agent of scrub typhus, is noted for its diversity with regard to both serotype and virulence. The serotypic diversity confounds the establishment of protective immunity to scrub typhus and complicates its serodiagnosis. The molecular basis for serotypic variation will be examined by identifying the species- and strain-specific epitopes of the major outer membrane protein immunogens, using monoclonal antibodies as probes. Because the proteins probably are important in other aspects of pathogen-host interactions as well, some biological activities of the rickettsial surface proteins also will be studied. The goals of this study will be accomplished by (1) determining the number and strain specificities of epitopes on the proteins by competitive ELISA tests, (2) determining which epitopes are on the rickettsial surface by immunofluorescent and ferritin-tagged antibody and electron microscopy, (3) assessing which epitopes are recognized during infection, and (4) attempting to neutralize certain rickettsial biological functions with selected monoclonal antibodies. Thus, these studies will determine the significance of the outer membrane proteins of R. tsutsugamushi in the antigenic variation of this species of intracellular pathogens and will begin to explain their role in rickettsial virulence. The proposed studies should form the foundation for further investigations of such questions as to the determinants of virulence of obligately intracellular pathogens, which factors play a role in protective immunity, which scrub typhus rickettsial components can best be used to prepare immunoprophylactic and immunodiagnostic reagents, and which aspects of R. tsutsugamushi surface structure are important to study in further analyses of rickettsial strains and isolates.